Rotary grinding ball mill



March 31, 1953 E, H, CUMPSTQN, JR f 2,633,303

ROTARY GRINDING BALL MILL y x J w.

l .I m Il I u I 2J @n l :I l I 'l I l INVENToR.

March 31, 1953 E. H. cUMPsToN, JR 2,633,303

Y ROTARY GRINDING BALL MILL'. Filed Maron 4, 1949 s sheets-sheet 2 March31, 1953 E. H. cUMPsToMJR' .2,633,303

ROTARY GRINDING BALL MILL l 3 Sheets-Sheet 5 Filed March 4, 1949 my M27/l l my. of# 4.144

INVENTOR. W'f.

(RPM.

Patented Mar. 31, 1953 ROTARY GRINDING BALL MILL Edward H. Cumpston,Jr., Lunenburg, Mass., as-

signor to Union Machine Company, Fitchburg, Mass., a corporation ofMassachusetts Application March 4, 1949, Serial No. 79,615

7 Claims.

This invention comprises a new and improved process of comminution orgrinding and includes Within its scope a novel rotary mill by which theprocess may be carried out.

In operating a ball mill the speed of rotation should be as high aspossible without causing the charge to pack on account of itscentrifuging tendency. As the speed of the mill increases, Work inputand grinding efficiency increase at first in proportion to the speed.Then as packing and centrifuging begin and increase, the work inputincreases more slowly than the speed of rotation until a critical speedis reached at which the charge begins to cling centrifugally to theshell of the mill. When this condition is reached, grinding efciencyfalls o rapidly, the charge merely whirls with the mill and grindingceases. This critical speed is determined by the equation w/(S-S) whereS is the radius of the mill and s the radius of the balls both expressedin feet. 'Ihe combined tendencies of centrifuging and packing limitpractical mill R. P. M. to the range of 60 to 80% of this criticalspeed.

I have discovered that by intermittently interrupting or retarding therotation of a charge of balls mixed with the material to be ground, I amable to operate substantially above the critical speed as abovedetermined, to utilize greater energy and achieve greater grindingeiciency than heretofore. By interrupting or momentarily reducing thespeed of rotation of the surface carrying the charge, its centrifugaleffect is reduced and also internal agitation of the charge isincreased. The combination of these two effects produces an entirely newand advantageous pattern in the action of the charge.

The novel process of my invention may be carried out with various formsof apparatus constructed and arranged to impart to the charge thedesired movement. That best suited for the purpose now known to mecomprises a drum driven in rotation about a fixed horizontal axis andhaving a multiplicity of weights movable upon the sides thereof in fixedopposed direction having components of motion in a plane perpendicularto the axis of the drum and around said axis, thereby alternatelyaccelerating and retarding the rotation of the drum surface carrying thecharge. In a preferred form of the mill the weights may be mounted onshafts external to the drum in diagonally opposite or equally spacedlocations and these shafts may be rotated at a substantially higher R.P. M. than that of the drum itself. Thus the centrifugal force of theweights tends alternately to retard and accelerate the rotation of thedrum. This construction has the advantage of being compact andselfcontained and of being effective independently of the drum drivingmechanism. It also serves as a mechanical structure capable of handlingthe large forces involved by correct engineering principles.

An optional feature of the invention consists in forming the drum withthe solid peripheraI wall presenting internal and external polygonalfaces thus providing at `their lines of intersection within the drumlifting angles for the contents of the mill. By this construction Ieliminate the necessity for internal lifting shelves or vanes within the`drum which are not only expensive to install but subject to excessivewear in operation.

These and other features of the invention will be best understood andappreciated from the following description of a grinding mill ofpreferred construction embodying the invention, useful in carrying outthe process thereof, and shown in the accompanying drawings, in which:

Fig. 1 is a View in elevation of the complete mill,

Fig. 2 is a corresponding View in end elevation,

Fig. 3 is a diagrammatic view of the other end of the mill partly incross section,

Figs. 4 and 5 are fragmentary views, partly in longitudinal section,illustrating the supporting and driving mechanism,

Fig. 6 is a diagrammatic view suggesting the position of the chargewithin the drum, and

Fig. '7 is a graph showing the relationship of drum rotation to speed ofgrinding.

It will be understood that the present invention is applicable not onlyto grinding operations carried out in ball mills, but also to grindingoperations carried out in pebble, rod or tube mills or, in fact, in anyprocess where the charge is rotated as a mass. For purposes ofillustration a novel ball mill suitable for my improved process will nowbe described.

The mill comprises a ydrum Ill herein shown as octagonal, but which maybe cylindrical, conical or polygonal in shape, or it may be anycombination of these shapes. It is mounted for rotation on hollowtrunnions I I and I 2 about a xed horizontal axis. The trunnions aresupported by bearings carried upon posts or standards I3 and I 4 risingfrom a suitable base or foundation. The

trunnions are hollow so that the material to be ground may be introducedto the drum and removed therefrom through them.

As herein shown the drum is driven by a pulley I which is secured to thetrunnion l2 beyond the standard I4 and about which passes a belt I6connecting the mill with the motor I1. A yielding connection is madebetween the pulley I5 and the trunnion I2 as best shown in Fig. 5. Thisincludes a sleeve I8 which is keyed to the trunnion and held in place bya ring I9 fast upon the end of the trunnion. The sleeve I8 is slotted atopposite sides to receive the inner ends of fiat bar springs 2i) whichare secured to bosses projecting outwardly from the hub of the pulleyI5. This connection provides a cushion between the drum and its drivingmotor in that the springs 20 are free to yield slightly to the shock ofany acceleration or retardation in the rotation of the drum.

The drum is provided on opposite sides with bearing brackets 2| in whichare journaled longitudinally disposed shafts 22. These'shafts extendbeyond the opposite heads` of the drum as best shown in Fig. l, and eachshaft is provided with a pair of weighted arms 23 which are arranged toswing in paths which clear the heads of the drum. Each shaft 22 isprovided at one end with a pinion 24 and these pinions mesh with a largestationary ring gear 25 mounted concentrically with the trunnion I2andsupported from the stationary bearing 26 thereof. The mounting of thering gear is best shown in Fig. 4 from which it will be seen that ashouldered collar 2l' is keyed to the inner end of the bearing 26 andprovides a circular seat for the ring gear which is held therein by aretaining ring 23 bolted to the collar 2. The collar is slotted atopposite points to receive the inner ends of bar springs 29 which extendthrough bosses 3l) projecting outwardly from the ring gear 25. The gearis thus held stationary but permitted to yield when the springs 29 areflexed to cushion shocks imparted thereto by the unbalanced rotation ofthe weighted arms 23, and non-uniform mill rotation.

The weighted arms 23 are secured to the shafts 22 in opposed phaserelation, that is to say, when the weighted arms carried by the shaft atthe lower side of the drum extend downwardly, the weighted arms carriedby the shaft at the upper side of the drum extend upwardly. The ratio ofdrum revolution to weighted arm revolution may be determined inaccordance with the dimensions of the ball mill, the size and shape ofthe weighted arms, and the formula for critical speed above given. Forexample, the drum may have a diameter of 15 inches to 6 feet and may berotated at 80 to 30 R.. P. M. with the shafts 22 geared to rotate at veto ten times the speed of the drum.

In operation the mixed charge of balls and material to be ground ispicked up by the internal surface of the drum and spilled in a cascadewithin some such zone as that indicated by the lines 33 and 34 in Fig.6, depending upon the speed of rotation of the drum. The effect of therotating weighted arms 23'is to accelerate the movement of the drum ormore specifically the linear speed of the drum surface,` duringapproximately 180 of their rotation while moving in advance oftheirshafts 22, and to correspondingly retard the motion of the drum whilemoving through approximately 180 of their path located behind the shafts22 relative to their direction of drum rotation. The accelerating andretarding effect of the weighted arms is imposed upon the otherwiseuniform rotation of the drum imil D parted to lt by the motor I1, and asalready explained, the intermittent intervals of retardation ordeceleration break up the centrifugal tendency of the charge so that asubstantial increase in grinding efficiency is secured when the mill isoperated at or above the critical speed indicated in Fig. 7. Forexample, excellent results have been secured in operating a 14 inch ballmill at 113% of the critical speed indicated by the formula.

Zin the graph of Fig. 7 the solid line curve shows the rate of grindingin pounds per minute of the material ground in relation to the R. P. M.of a conventional ball mill driven uniformly. The line 32 indicates thecritical speed calculated by this formula and the normal and usual zoneof operation, i. e. 60 to 80% of the critical speed, as indicated byreference character 35. 'I'he dash line curve shows the same relation ofgrinding rate to R. P. M. of a mill operated with intermittentdeceleration in accordance with the process of my invention. In thiscase the zone of operation is increased to to 140% of the criticalspeed, as indicated by reference character 35, and the rate of grindingof the mill continues to increase as the R. P. M. of the mill isincreased to 140 or more of the otherwise critical speed. y

The increase in efficiency of the grinding operation carried out inaccordance with the present invention is due in part to the fact thatthe dead spot or Zone of the charge is substantially eliminated. It willbe understood that when a ball mill is operatedr at a uniform speed, adefinite zone is formed in the charge in which the balls remainsubstantially at rest with respect to each other and no grinding occurs.It has been found that this deadv spot disappears when the charge isintermittently retarded so that its centrifugal effect is reduced, thecharge caused to cascade under the influence of gravity more freely andthe area of the grinding zone is substantially increased. For example,it may be increased in area from the line 33 to the line 33' assuggested in Fig. 6.

The retarding and accelerating effect of the weighted arms 23 is themore pronounced because of the fact that the effective torque of thearms in the direction of drum rotation is neutralized twice in eachrotation of the weights. This occurs when the oppositely disposedweights instantaneously extend radially outwardly or radially inwardlywith respect to the axis of rotation of the drum. It is in movingbetween these two neutralizing positions that the accelerating anddecelerating effect of the weighted arms becomes apparent.

The torsional input of the weighted arms is sinusoidal in character andpasses through zero twice in each revolution of the arms. The arms as asystem balance each other in all positions with respect to the bearingsof the mill, a condition which permits the employment of a much lighterconstruction and insures longer life of t the apparatus than would bethe case if the system were unbalanced. In the illustrated constructionthis balance issecured by locating the shafts 22 in diagonally oppositepositions on the drum but it will be apparent that the desired balancewould be secured by any circumferentially equal spacing of weights; forexample three weights spaced apart.

While balls have been referred to for convenience as the grindingelements with which the charge is made up elements of any other shapeare to be considered as the full equivalent of balls for purposes of thepresent invention. It would also be within the scope of the invention toconstruct and arrange the drum for rotation about a somewhat inclinedaxis or to tilt the axis of the drum.

In carrying out the present invention it is convenient in many cases toemploy the standard driving mechanism of commercial ball mills and thisusually includes a gear in place of the pulley I5 herein shown. Whethera pulley or gear is employed the yieldable connection, herein shown asthe bar springs 20, permits an angular slope of about five or sixdegrees, for example, on either side from normal driving position atuniform rate. The term rate is used herein as meaning R. P. M. It willbe observed that there is substantially no loss in driving energy sincethe force used in each rearward deflection of the spring bars is appliedin accelerating the drum during the ensuing phase of the eccentricweights. In practice the retarding force of the eccentric weights iscalculated so that the rotation of the drum is never completelyarrested.

Having thus disclosed my invention and described in detail anillustrative embodiment thereof, I claim as new and desire to secure byLetters Patent:

l. A ball mill or the like comprising a drum, stationary bearingssupporting the drum for rotation about a fixed horizontal axis, drivingmeans connected to the drum for rotating the same, shafts mountedexternally upon the drum parallel to the axis thereof in diagonallyopposite locations, means for rotating said shafts at a higher speed ofrotation than the drum, and eccentric weights carried by the shafts andset in opposed phase relation to each other.

2. A ball mill or the like comprising a drum, stationary bearingssupporting the drum for rotation about a fixed horizontal axis, meansfor rotating the drum, a series of weights movably mounted upon the drumfor motion having components in a plane perpendicular to the axis of thedrum and around said axis, and means for moving said weightssimultaneously rst in the direction of rotation of the drum and thenoppositely to its direction of rotation.

3. A ball mill or the like comprising a drum, stationary horizontalbearings supporting the drum for rotation about a xed axis, eccentricweights mounted for rotation about axes parallel to the axis of the drumon the drum, means for rotating the drum, and mechanism including ayieldable element for rotating the weights.

4. A ball mill or the like comprising a drum, stationary horizontalbearings supporting the drum for rotation about a xed axis, amultiplicity of eccentric weights mounted for rotation on the drum,connections including a yieldingly mounted gear for rotating the weightson the drum, and other connections including a yieldingly mounted pulleyfor rotating the drum.

5. A ball mill or the like comprising a drum, stationary horizontalbearings supporting the drum for rotation about a xed axis, shaftsmounted on opposite sides of the drum, unbalanced weights carried byeach shaft in spaced apart and in opposed phase relation, a pinion oneach shaft, and a normally stationary but yieldingly mounted gearmeshing with each of said pinions and acting to rotate said shafts andWeights as the drum is revolved.

6. A ball mill or the like comprising a drum having hollow trunnions,stationary bearings for said trunnions whereby the drum is supportedthereby for rotation about a fixed horizontal axis, a multiplicity ofshafts mounted upon the drum in pairs apart, each shaft carryingunbalanced weights at both ends arranged to swing in paths beyond theheads of the drum, a pinion fast to each shaft beyond the weight at oneend thereof, and a yieldingly mounted ring gear disposed with clearanceabout one trunnion of the drum and meshing with the said pinions.

7. A ball mill or the like comprising a drum. stationary bearingssupporting the drum for rotation about a xed horizontal axis, means forrotating the drum, rotary eccentric weights carried by the drum andlocated with equal circumferential spacing, and means for rotating thesaid weights at a higher rate than that at which the drum is driven.

EDWARD H. CUMPSTON, JR.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 569,828 Herzfeld Oct. 20, 1896858,930 Warrington July 2, 1907 1,449,845 Taylor Mar. 27, 1923 1,860,383Chalmers May 31, 1932 2,014,640 Wales Sept. 17, 1935 2,284,548 Wood May26, 1942 FOREIGN PATENTS Number Country Date 233,164 Germany Apr. 1,1911 633,699 Germany Aug. 4, 1936 540,816 Great Britain Oct. 30, 1941231,753 Switzerland July 17, 1944

